1. Field of the Invention
The present invention relates to a laser radar for irradiating laser light to a target area, and detecting a state of the target area based on a receiving state of the laser light reflected on the target area, and a beam irradiation device suitably used in the laser radar.
2. Description of the Related Art
In recent years, a laser radar for detecting a state of a target area by irradiating laser light to the target area has been loaded in a family automobile or the like. The laser radar is configured in such a manner that the target area is scanned with laser light to detect an obstacle (i.e. an object to be detected) based on a receiving state of reflection light from the target area, and at the same time, the distance to the obstacle is measured.
A beam irradiation device is incorporated in the laser radar as an arrangement for scanning the target area with laser light. The target area is scanned with laser light by using e.g. a lens actuator. A scan lens is two-dimensionally driven in a direction perpendicular to the optical axis of laser light in accordance with driving of the lens actuator. Thereby, the target area is scanned with laser light.
The beam irradiation device may have an arrangement for increasing the swing width of laser light. For instance, an attachment lens (i.e. a scan expansion lens) is provided in front of the scan lens. The attachment lens is advantageous in further increasing the swing width of laser light to be generated by displacing the scan lens.
The laser radar has a detector for receiving reflection light from the target area. The detector receives reflection light from an obstacle at a scan position of laser light, and outputs a detection signal. The detector may be constituted of e.g. a light receiving element such as a photodiode, and a condenser lens for condensing reflection light from the obstacle and guiding the condensed light to the light receiving element. The condenser lens has a lens surface facing the target area in forward direction to receive the reflection light from the target area.
In the case where the attachment lens is provided with the detector, the attachment lens and the condenser lens are arranged at such a position that the directions of the lens diameter thereof are aligned with each other.
The condenser lens has a significantly large size in the lens diameter direction thereof to receive reflection light from the target area in a wide range. As a result, the detector has a large size in the lens diameter direction of the condenser lens. On the other hand, a light receiving surface of the light receiving element for receiving reflection light from the target area has a considerably small size, as compared with the lens diameter of the condenser lens. As a result, a large dead space is defined between the condenser lens and the light receiving element. The dead space obstructs miniaturization of the beam irradiation device and the laser radar. Also, since the lens diameter of the condenser lens is large, there is a problem that the size of the beam irradiation device and the laser radar may be increased in the lens diameter direction.
In the laser radar, generally, the distance to the obstacle is measured based on a difference in time between an emission timing of laser light, and a receiving timing of reflection light from the target area. In this arrangement, the time difference is measured based on an output timing of a signal (i.e. an emission signal) indicating emission of laser light, and an output timing of a signal (i.e. a light receiving signal) indicating receiving of reflection light from the light receiving element. In this arrangement, there is a time lag between the point of time when the emission signal is outputted, and the point of time when the laser light is actually emitted. There is also a time lag between the point of time when the light receiving signal is outputted in response to actual receiving of reflection light, and the point of time when the light receiving signal reaches a processing circuit.
In a circuit system, generally, a time difference between the emission timing of laser light, and the light receiving timing of reflection light from the target area is measured, taking the aforementioned time lags into consideration. However, there is a case that the aforementioned time lags include an error resulting from a characteristic of a laser light source or a light receiving element. If the error is included, the time difference between the emission timing of laser light, and the light receiving timing of reflection light from the target area may include an error. As a result, measurement precision of the distance to the obstacle may be lowered.